This invention relates to aqueous inks for ink jet printers, and more particularly, to aqueous ink jet inks containing a linear polymer as an additive to improve smear resistance, increase color gamut and reduce bleed of the applied ink.
Ink jet printing is a non-impact printing process in which droplets of ink are deposited on a substrate such as paper or transparency film in response to an electronic signal. Low cost and high quality of the output, combined with relatively noise free operation, have made ink jet printers a popular option to other types of printers used with computers.
Both dyes and pigments have be en used as ink colorants for ink jet printers. The dye-based inks while in general superior in color properties have several disadvantages as compared to pigment based inks. The dyes are usually water soluble and remain so after drying on the substrate. They are easily redissolved by water spills, and the image smears on contact with felt pen markers. In addition, the dyes exhibit poor light stability relative to pigments and are known to fade even under office lighting. Thus, dye-based inks are often unsuitable for use in applications requiring moisture resistance and greater light stability. The pigments are preferred colorants provided the pigment dispersion can be made resistant to flocculation and settling.
Water-based pigment dispersions are well known in the art, and have been used commercially for applying films, such as paints, to various substrates. The pigment dispersion is generally stabilized by either a non-ionic or ionic technique. When using the non-ionic technique, the pigment particles are stabilized by a polymer that has a water-soluble, hydrophilic section that extends into the water and provides entropic or steric stabilization. Representative polymers useful for this purpose include polyvinyl alcohol, cellulosics, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers. While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage for many applications in that the final product is water sensitive. Thus, if used in ink applications or the like, the pigment will tend to smear upon exposure to moisture, and mechanical action such as handling. "Highlighting" text with a felt-tip pen which contains translucent ink is particularly damaging to the image, since the solvents in the highlighting ink smear the text, and the mechanical action smudges the print. Also, pigmented ink systems tend to have low chroma versus dye-based ink systems, and are prone to poor pen performance due to ink crusting on the pen orifices, coating of the pen resistors, etc.
There exists a need for ink jet inks which have good water resistance, smear fastness, higher chroma and good pen performance. In particular, with pigment based inks there is a need to maintain the stability of the dispersion while improving water and smear fastness.